Abstract The age someone feels predicts important outcomes in health, cognition, and general well-being, such as depressive symptoms and chronic illness, above their actual chronological age. Thus, determining the daily contexts associated with changes in subjective age may help us find ways to promote younger subjective ages and better heath. The proposed study will use ESM (experience sampling methodology) to capture fluctuations in subjective age during daily life and connect these fluctuations to their specific contexts. For example, we expect that experiences that highlight an individual's age, such as completing a task that is perceived to be challenging for older adults, will elevate subjective age, particularly when a participant encounters difficulty completing the task. Past work has demonstrated change in subjective age following a brief intervention. After taking a memory test, which older adults find cognitively stressful, they reported older subjective age. Taking a vocabulary test did not increase subjective age. The proposed study will integrate a similar experimental manipulation within the ESM protocol to examine the impact of targeted cognitive stressors on changes in subjective age during daily life. Participants will be provided with smartphones for seven days. On each of these seven days, the cell phone will randomly prompt the participant to answer a short series of questions regarding their current activities. Next, participants will either be prompted to complete a short memory test, vocabulary test, or no test. Finally, participants will be asked four questions about their subjective age. Specific Aims: (1) Test the hypothesis that subjective age changes not only over years and day-to-day, but fluctuates moment-to-moment throughout the course of a day. (2) Clarify how situational contexts influence fluctuations in subjective age. We hypothesize that subjective age will fluctuate due to predictable factors such as time of day, with older felt age at later times. We also expect that task-specific factors such as engagement and confidence lead to younger felt age, while other task factors such as difficulty lead to older felt age. (3) Examine the specific impact of stressful cognitive experiences on subjective age during everyday life. We predict that completing a cognitive task with stereotypes of age-related decline, such as a memory test, will elevate subjective age, compared to a less stressful (i.e., vocabulary test) or no cognitive experience. Significance: This research will provide foundational knowledge regarding whether subjective age fluctuates throughout the day, and an understanding of how situational variation impacts subjective age, both as it naturally occurs and in a targeted manipulation of cognitive challenge. Given that subjective age may impact active lifestyle choices, this study can inform interventions that benefit health, cognition, and well-being. Innovation: This research will investigate subjective age in older adults concurrently with daily life activities as opposed to in a once-daily retrospective evaluation, the first such study to do so. The manipulation of cognitive stress, multidimensional assessment of subjective age, and the slider approach are also innovative.